1. Field of the Invention
Centrifugal and other liquid pumps, and provisions to prevent air locking of such pumps.
2. Description of Related Art
A pump is a device used to transport liquid from a lower to a higher elevation, or from a lower pressure state to a higher pressure state. Typically, an electric motor is used to spin an impeller inside a volute casing transferring energy to the liquid. In many instances, a pump is submerged in a reservoir of liquid, and its discharge is connected to a pipe that is used to convey the liquid to a higher elevation or higher pressure vessel.
A backflow prevention device such as a check valve is used to prevent the flow of liquid through the pump, once the pump has stopped. The pump starting and stopping is typically operated by a device such as a float switch, and is turned off prior to the pump sucking air. Under normal operation, the inlet of the pump is submerged and is never exposed to air during its operation.
However, malfunctions do sometimes occur, and in such circumstances, if the pump is not turned off and does suck air, then a problem may occur the next time the pump is restarted. When the check valve closes, a column of water sits on top of its mechanism and upon restarting, the pump must develop sufficient head to displace the mechanism and the liquid above it enough to resume pumping. A liquid pump is not designed to move air, and the presence of air in the volute chamber will greatly reduce the pumping performance of the pump. Under these circumstances, the pump will not develop adequate head pressure to overcome the closing force of the check valve and the static force of the liquid column acting on it. The pump impeller will spin, but no liquid will be discharged from the pump. The check valve will simply remain closed, with the liquid column above the check valve remaining motionless. A pump in this condition is referred to as being “air locked.”
Many pump designs have a bleed hole in the volute wall that allows air to be expelled back into the reservoir and replaced with liquid drawn in through the pump intake. When the liquid being pumped is free of solid debris, this is quite effective. However, when solid debris, such as solids from macerated toilet effluent, or from a sewage grinder pump is suspended in the liquid, the bleed hole can become blocked with debris particles. This renders it ineffective. Many pump manufacturers suggest cleaning this hole routinely to maintain its effectiveness. However, in some installations, this is not practical from a cost or time standpoint. Additionally, for a sewage grinder pump, or a pump that is part of a macerating apparatus for a toilet, because of the unsavory contents of the liquid, this is an unpleasant maintenance task that is to be avoided.
There is therefore a need for a pump with anti-airlock capability, which capability is not disrupted by the presence of solid materials in the liquid stream being pumped.